Thermosetting resins such as epoxy resins, urethane resins and phenolic resins possess excellent heat resistance and mechanical strength and have therefore been used extensively, for example, as a molding material for electric and electronic parts, constructional materials and automobile parts, a coating, an adhesive, and an impregnating resin for impregnated paper. Flame retardation of these thermosetting resins has been achieved generally by addition of a halogen-containing organic compound or by halogenating part of the polymer chain. However, halogen-containing thermosetting resin compositions generate a corrosive halogenous gas on combustion, which has given rise to a problem. Ammonium polyphosphate has been proposed as a halogen-free flame-retardant, but it is very susceptible to hydrolysis because of its chemical structure so that molded articles obtained from a resin composition containing ammonium polyphosphate suffer from elution or bleeding of the ammonium polyphosphate under high temperature and high humidity. It has also been pointed out that compounding of such an inorganic compound as ammonium polyphosphate leads to reduction of mechanical properties of the resin composition. It has therefore been demanded to develop a flame-retardant thermosetting resin composition containing a flame-retardant which produces no halogenous gas on combustion, is not eluted, does not bleed out, and causes little reduction in mechanical characteristics of the resin.
In order to meet the demand, a resin composition containing a metal hydrate as an inorganic flame-retardant has been proposed as disclosed in JP-A-3-190965 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), in which the metal hydrate undergoes an endothermic decomposition and dehydration reaction at the combustion temperature of the resin composition thereby suppressing combustion of the resin composition. However, since the flame retarding effect of the metal hydrate is very weak, desired flame retardation cannot be achieved unless it is used in a large quantity, and addition of a large quantity of the metal hydrate results in reduction in moldability of the resin composition and reduction in mechanical strength of molded articles of the resin composition.
JP-A-4-227923 discloses an epoxy resin composition which is flame retarded with a carbon-forming non-halogen type flame-retardant containing ammonium polyphosphate, but the ammonium polyphosphate used is conventional one which is highly water-soluble and easily hydrolyzable.
On the other hand, JP-A-1-108261 discloses a flame-retardant polymer composition containing synthetic resin-coated ammonium polyphosphate particles, and JP-A-5-9376 describes a flame-retardant urethane resin composition containing ammonium polyphosphate particles which are encapsulated by a melamine-formaldehyde resin or an epoxy resin. In these compositions, since the ammonium polyphosphate particles are coated or encapsulated with a synthetic resin, it seems that ammonium polyphosphate is prevented from being eluted and thereby from bleeding, but the publications make no mention of it. Further, the synthetic resin coat covering the individual ammonium polyphosphate particles is a cured resin and is no longer reactive. Therefore, when added to a thermosetting resin, such a synthetic resin coat has poor compatibility with the thermosetting resin so that the resulting flame-retardant resin composition only provides molded articles having reduced mechanical strength.
With respect to improving the stability of ammonium polyphosphate to hydrolysis, JP-B-53-15478 (the term "JP-B" as used herein means an "examined published Japanese patent application") discloses a process for obtaining modified ammonium polyphosphate comprising condensing ammonium phosphate and urea or crystalline urea phosphate while heating in the presence of 5 to 50% by weight, in terms of melamine, of melamine or melamine phosphate. However, the modified ammonium polyphosphate obtained by the process disclosed still has insufficient stability against hydrolysis for use as a component of a flame retardant for thermoplastic resins, thermosetting resins, coatings, flame-retardant paper, etc.
JP-B-52-39930 discloses a process for obtaining melamine-added ammonium polyphosphate powder comprising uniformly mixing 100 parts by weight of ammonium polyphosphate and 60 parts by weight of melamine, heating the mixture at 320.degree. C., cooling the heated product, and grinding the resulting fused product. However, because a product having been once fused is ground to powder, the resulting ammonium polyphosphate particles are not uniformly coated with melamine and are, therefore, still susceptible to hydrolysis.
JP-A-61-103962 proposes a process for producing a finely divided and flowable flame-retardant mainly comprising powdered ammonium polyphosphate having hydrolysis stability, which process comprises preparing a suspension from ammonium polyphosphate, water, an organic suspending agent, and a melamine formaldehyde resin, heating the suspension at a temperature of 50 to 180.degree. C. under standard pressure or under a higher pressure, and leaving the suspension to stand for 15 to 240 minutes to allow the resin component to harden. However, not only does the finely divided flame-retardant obtained have insufficient hydrolysis stability, but the particles undergo agglomeration during hardening of the resin component in the suspension, resulting in an increase of particle size. Therefore, no matter how fine the starting ammonium polyphosphate particles are, the initial particle size of the ammonium polyphosphate can hardly be maintained in the resulting flame-retardant. Such a finely divided flame-retardant, when added to thermoplastic resins, thermosetting resins, coatings or paper, causes reduction in mechanical strength of the final products.